1. Field of the Invention
The present invention relates generally to a sensor for use in high pressure and high temperature applications, such as in an oil/gas well, and more particularly to a fiber optic based hydrophone.
2. Description of the Related Art
Fiber optic based acoustic sensors, such as hydrophones, are known in the art for use in a number of applications involving the sensing of acoustic pressures in fluidic media (i.e., gas or liquid). Examples of prior art hydrophones are disclosed in the following U.S. patents which are hereby incorporated by reference in their entireties: U.S. Pat. Nos. 5,625,724; 5,317,544; 5,668,779; 5,363,342; and 5,394,377. A review of these references reveals the general structure of a fiber optic based hydrophone, which constitutes a winding of optical fiber or a fiber Bragg grating (FBG) wrapped around a compliant cylindrical mandrel. As is well known, when acoustic pressures in the fluidic media being measured impinge upon the mandrel, the mandrel will deform, thus perturbing the optical fiber. Optical detection of the change of the winding (e.g., by the use interferometric techniques), or optical detection of the Bragg reflection wavelength shift of the FBG, allows the impingent pressure to be quantified.
However, most prior art hydrophones are believed to be unsuitable for deployment in extremely harsh environments, such as those present within an oil or gas well. As one skilled in the art will understand, the downhole environment of an oil/gas well is characterized by extremely high pressures (e.g., 15 k psi) and temperatures (e.g., 150-250 Centigrade). Furthermore, the downhole environment also potentially contains caustic chemicals and bits of debris. The design of the above-referenced hydrophones would likely be damaged by deployment down an oil/gas well, as they disclose designs that would either collapse under the extreme pressures of the downhole environment, would be degraded by the high temperatures and chemicals that exist downhole, or would be damaged by debris that could potentially contact the exposed delicate windings of fiber optic cable present in some of these designs. Additionally, as some of these prior art designs rely on the use of non-compliant reference windings to supplement the compliant measurement windings, the extreme pressures present downhole could affect the reference windings, thus rendering the referencing scheme unreliable for downhole use. Furthermore, at least some of these designs are not suitably small in size for deployment down the well, e.g., within the annulus of the well between the production pipe and the well casing cemented to the borehole of the well.
Therefore, a need exists in the art for an acoustic sensor and more specifically a fiber optic based hydrophone that is relatively small and is capable of operating at high temperatures and/or pressures, and that is not susceptible to caustic downhole chemicals.
(For further reference concerning hydrophone technology, the reader is referred to the following U.S. patent applications, which are incorporated herein by reference: Ser. No. 10/348,445, filed Jan. 21, 2003, and Ser. No. 10/393,170, filed Mar. 20, 2003).